On board cargo ventilation systems for roll on-roll off cargo spaces generally operate according to the principle of dilution ventilation. Dilution ventilation applies where the supply air flow to the area is sufficient for the exhaust gases to mix thoroughly with the air and be removed. Dilution ventilation occurs in two main types: exhaust air ventilation and supply air ventilation. Briefly, in exhaust air ventilation, fans remove air from a cargo space. This is then replaced by outdoor air entering through open ramps, doors and other openings. Exhaust air ventilation is employed when the pressure required is lower than that of the atmosphere. This low pressure prevents the pollution from spreading to the adjacent areas.
Working in the opposite way is supply air ventilation. Fans deliver outdoor air into the cargo space and the air is then exhausted through ramps and other openings. Slight pressurisation of the cargo space is usually created when using supply air ventilation. When used exclusively, in supply air ventilation, pollutants may mix with the supply air or be pushed up the internal ramps and contaminate other decks. However, if sufficient mixing with supply air does not occur, contaminants may remain on deck and particularly hazardous conditions may occur on lower decks.
Cargo ventilation systems on board ship often combine these two principles. The fans can then be reversible, so that they can either supply air into the ro-ro cargo space or exhaust air from it. The function of a ventilation system in the roll on roll off cargo space is to dilute and remove the vehicle exhaust gases and other hazardous gases, to protect people working in the area from being exposed to a hazardous or disagreeable level of air pollution.
When a new ship is put in operation, the air quality should be tested by a competent qualified person with specialist training in occupational exposure. The tests should be carried out in consultation with the ship’s safety delegate and any other relevant authorities. Vessel owners should consider testing the air quality in conjunction with tests of the ventilation system to ensure proper maintenance and functioning of the ventilation system. Situations which indicate the necessity to conduct air quality monitoring include worker complaints including headache, dizziness, stinging of the eyes or respiratory system, in addition to indications that the ventilation system itself has deteriorated, and changes in vessel operation which are substantially different from that for which the original ventilation system was verified. All tests results verifying the adequacy of the ventilation system should be documented and kept with the ship’s records.
Industrial fans for use within dockland applications must be suitable for the cause. Often requiring robust equipment that can cope with a ships potential environments and change in electrical environments are important considerations.